In the evolving world of cutting tools, desirable cutting inserts include those tools in which the edges are superhard and processes such as brazing in the manufacture of the finished insert are eliminated. This requirement is partially fulfilled by fully solid inserts formed from a homogenous monolithic body of superhard materials. Normally, these inserts allow cutting edges on both the top and bottom surfaces of the insert, contributing to the economic benefit of their use. In other words, they tend to be used mostly in “negative” geometries where the side faces of the inserts are perpendicular to both the top and bottom of the insert.
However, in a number of scenarios, “positive” inserts are required, i.e. the side faces are not perpendicular to either the top or the bottom of the insert and cutting edges forming only acute include angles are usable. In such inserts, the acute angled cutting edges occur on the top of the insert adjoining the hard-layer. For the raw material of the cutting tool, it is more economical to use layered superhard grades. Here, only the top layer, typically 0.5-2.0 mm thick, comprises the superhard material. The remaining portion in the bottom is composed of tungsten carbide/cobalt composites. The superhard layer is integrally bonded to the carbide layer during the sintering process itself.
An important difference between the two layers, aside from the hardness, is that the carbide layer is more easily machined with electrical discharge processes. This raw-material design removes attendant redundancy of PCBN in the fully solid inserts.
In “positive” inserts, a clamping hole in the insert becomes necessary to locate the insert in the tool-holder or cartridge pocket opposing the cutting forces. Furthermore, the sides of such inserts are required to be ground to the required relief angles. The relief angles create the positive rake angles and the acute included angles at the cutting edge.
To achieve this, current practice uses a layered superhard tip brazed in a pocket ground in a carbide insert and is subsequently ground to final dimensions. The clamping hole in the carbide insert serves to locate the insert in a tool-holder or cartridge pocket. Therefore, the amount of superhard material to be ground is only of the order of the lateral dimensions of the tip and not of the insert dimensions itself.
This process design places severe restrictions. For example, more aggressive cutting conditions may demand a larger tip be brazed in the insert. However, the space available for the tip in the carbide insert itself may itself be small. Another likely scenario is that cutting temperatures in the tip are high enough to cause de-brazing of the tip, for example, in titanium machining.
A solution to these problems is an insert with an integral clamping hole. Such an insert also increases the number of usable edges from a single tip to tips at every corner on the top face. It also allows for more compactness. For example, in a milling cutter, the integral clamping hole allows more inserts to be stacked for a given diameter, because of the room gained by eliminating top-clamps. A greater number of inserts in the cutter would allow greater feed-rate of cutting and consequently greater productivity.
However, an insert with an integral clamping hole dictates that the amount of superhard material to be ground is of the order of the insert dimensions itself, likely resulting in higher grinding cost and time. Several steps are required to reduce this cost: 1) the raw insert presented to the tool-grinder is as close to the final desired shape and dimensions as feasible; and 2) the amount of grindstock on the insert is reduced down to the depth of subsurface damage caused by insert severing processes such as WEDM and laser. To achieve this, the manufacturing process to produce this raw insert is designed to eliminate all geometrical form errors.
The three key geometrical criteria are a) perpendicularity of the axis of the integral clamping hole to the insert top, b) concentricity of the through-hole to the inscribed circle of the insert and c) proximity of edge damage on the through-hole entrance. Criteria a) and b) are important to ensure that the amount of grindstock on each side of the insert is the same. Criterion c) is important to ensure that an adequate amount of superhard material is available for incorporating a suitable chamfer and/or hone to the cutting edge and the integrity of the insert itself. Both a) and b) may be eliminated if the operation of severance of the insert and finishing of the through-hole are performed in the same setup on the same WEDM machine. Criterion c) may be eliminated if the process of obtaining the profile of the hole involves only WEDM and not electrical discharge with an electrode.
If the severance of the raw insert from the polycrystalline body is performed such that the insert sides bear the relief angles of the final finished ground insert, significant savings in grinding cost and time are obtained, since part of the superhard material removed in the tool grinding process is removed by the WEDM without impacting the finished cutting tool in any way.
In other words, it is important to exploit the flexibility of the WEDM process fully, since it represents very minimal and localized damage to the superhard material compared to electrical discharge processes using an electrode. The ability to tilt the wire while cutting superhard material is a significant facility which until now has not been utilized in the manufacture of superhard inserts with integral clamping holes.
However, one difficulty is that when surfaces cut with the wire are such that the severed part is concave upwards, there is a tendency of the severed part to drop by gravity and produce an electrical short of the wire. The manufacturing process design has to take this important factor into account.
There is a need for simplicity and uniform costs, to keep the manufacturing process standard for all grades of PCBN and PCD. The one process where different grades differ greatly is electrical discharge grinding using an electrode, though slight differences are likely in the WEDM process also. This need is particularly amplified when the lot size of inserts to be manufactured is rather large and on a recurring basis, for example in standard ISO specified inserts, unlike a one-off case of specialized geometry. Elimination of electrical discharge with an electrode on the hard-layer is the single biggest step towards achieving this goal. Such a process design vastly simplifies the finished-tool manufacturing process and provides economy of scale when inserts are produced in large quantities.